The present application relates to a memory devices. In particular, the present application relates to memory device having a capacitor structure.
The capacity of semiconductor memory devices such as Dynamic Random Access Memory (DRAM) or Static Random Access Memory (SRAM) is increasing steadily following Moore's law which predicts that the number of transistors on a chip doubles every 18 months. Manufacturers are making great efforts to maintain this rate of improvement. One of the major issues is how to fabricate a capacitor in a smaller area without reducing its capacitance, since in order to obtain reliable data retrieval large capacitance is required.
One solution is to form a deep trench on a silicon substrate and fabricate a capacitor in it so that the surface area, which is proportional to the capacitance, is increased. This structure is illustrated in FIG. 1(a), and the equivalent circuit diagram is shown in FIG. 1(b). A transistor (the region marked generally as 1) is formed on a substrate 7 with two of its terminals connected respectively to a word line 5 and a bit line 6. The third terminal of the transistor is electrically connected to an electrode 3 which is one of a pair of electrodes 2, 3 sandwiching a dielectric layer 4. The electrodes 2, 3, expend over the surfaces of a trench in the substrate 5. Unfortunately, this structure requires a complicated fabrication process that pushes up its production cost.
In separate developments, ferroelectric random access memory (FeRAM) is starting to be utilized for a certain category of products making use of its non-volatility. Since it does not require any electric power to maintain the stored data, it is suitable for use in passive devices, such as RF tags or IC cards. A single memory cell consists of a thin film of a ferroelectric material and a pair of electrodes which are located respectively on the top and bottom surfaces of said ferroelectric thin film. Information is stored as a direction of polarization induced in the ferroelectric film. The ferroelectric material itself may be organic or inorganic. In general, organic ferroelectric materials have the advantages of easier fabrication and inexpensiveness. However, the remanent polarization of organic material is not as large as that of inorganic material, so the readout signal is lower. Raising the read-out signal would be advantageous, to obtain more reliable data retrieval.